John:

Good analysis. I have a few additional thoughts based on limited use
of SPOT at SoaringNV here in Minden:

First, SPOT is a non-emergency tracker. One of our instructors
carries his personal SPOT on long XC flights and we can get his
current position (updated every 10 minutes or so) online. This means
we know if he is making normal progress or is spending a long time
somewhere looking for a thermal. It also lets us know if there's a
good chance he'll make it home without an aeroretrieve, so we can send
the tow pilot home early.

ELTs and PLBs don't offer this kind of capability. It's a
convenience, not an emergency consideration, but it is a
consideration.

Another aspect of ELT use (which does not, I don't think, apply to PLB
use) is that they are frequently triggered inadvertantly. I fly with
the local Civil Air Patrol, and I think more than half the alerts they
are called out on turn out to be to search for an ELT alarm that they
subsequently learn is in an aircraft sitting on an airport somewhere.
And this high incidence of false alarms brings to light how slow the
response actually is to a signal on 121.5. By the time the Sheriff or
CAP locates the triggered alarm it has often been alerting for a
number of hours. My understanding is that the NOAA satellite that
responds to 121.5 needs to get two signals from the same location
before it alerts, and this requires two passes of the satellite. This
might not be the case any longer, but I'm pretty sure it was when the
service was first put in place.

Finally, SPOT and PLBs both offer active alert capability, while ELTs
offer passive alert. I suppose this could be a consideration if you
are incapacitated upon reacing the ground -- maybe you couldn't push
the "help" button -- so you need to think about what you want to be
prepared for: letting people know you're ok, or letting the NOK know
where to find the remains. (Sorry for the gruesome touch.)

Fred